Illumination systems are used as either stand-alone light sources or as internal light sources for more complex optical systems. Examples of optical systems that utilize or incorporate illumination systems include projection displays, flat-panel displays, avionics displays, automotive lighting, residential lighting, commercial lighting and industrial lighting applications.
Many applications require illumination systems with high brightness and a small effective emitting area. An example of a conventional light source with high brightness and a small effective emitting area is an arc lamp source, such as a xenon arc lamp or a mercury arc lamp. Arc lamp sources may have emitting areas as small as a few square millimeters. An example of a complex optical system that can utilize an illumination system with high brightness and a small effective source area is a projection television display. Current projection television displays typically project the combined images of three small red, green and blue cathode-ray-tube (CRT) devices onto a viewing screen using projection lenses. More recent designs sometimes use a small-area arc lamp source to project images from a liquid crystal display (LCD), a liquid-crystal-on-silicon (LCOS) device or a digital light processor (DLP) device onto a viewing screen. Light sources such as LEDs are currently not used for projection television displays because LED sources do not have sufficient output brightness.
The technical term brightness can be defined either in radiometric units or photometric units. In the radiometric system of units, the unit of light flux or radiant flux is expressed in watts and the unit for brightness is called radiance, which is defined as watts per square meter per steradian (where steradian is the unit of solid angle). The human eye, however, is more sensitive to some wavelengths of light (for example, green light) than it is to other wavelengths (for example, blue or red light). The photometric system is designed to take the human eye response into account and therefore brightness in the photometric system is brightness as observed by the human eye. In the photometric system, the unit of light flux as perceived by the human eye is called luminous flux and is expressed in units of lumens. The unit for brightness is called luminance, which is defined as lumens per square meter per steradian. The human eye is only sensitive to light in the wavelength range from approximately 400 nanometers to approximately 700 nanometers. Light having wavelengths less than about 400 nanometers or greater than about 700 nanometers has zero luminance, irrespective of the radiance values.
In U.S. patent application Ser. No. 10/445,136, brightness enhancement referred to luminance enhancement only. Since luminance is non-zero only for the visible wavelength range of 400 to 700 nanometers, U.S. patent application Ser. No. 10/445,136 is operative only in the 400- to 700-nanometer wavelength range. In the present application, however, brightness enhancement refers to radiance enhancement and is valid for any wavelength throughout the optical spectrum.
In a conventional optical system that transports light from an input source at one location to an output image at a second location, one cannot produce an optical output image whose radiance is higher than the radiance of the light source. A conventional optical system 10 of the prior art is illustrated in cross-section in FIG. 1A. In FIG. 1A, light rays 18 from an input light source 12 are focused by a convex lens 14 to an output image 16. The conventional optical system 10 in FIG. 1A can also be illustrated in a different manner as optical system 20 shown in cross-section in FIG. 1B. For simplicity, the input source 22, the lens 24 and the output image 26 are all assumed to be round. In FIG. 1B, the input source 22 has area, Areain. The light rays from input source 22 fill a truncated cone having edges 21 and 23. The cone, which is shown in cross-section in FIG. 1B, extends over solid angle 27. The magnitude of solid angle 27 is Ωin. Lens 24 focuses the light rays to image 26 having area, Areaout. The light rays forming the image 26 fill a truncated cone having edges 25 and 29. The cone, which is shown in cross-section, extends over solid angle 28. The magnitude of solid angle 28 is Ωout.
If the optical system 20 has no losses, the light input flux at the input source 22,Φin=(Radiancein)(Areain)(Ωin),  [Equation 1]equals the light output flux at the output image 26,Φout=(Radianceout)(Areaout)(Ωout).  [Equation 2]In these equations, “Radiancein” is the radiance at the input source 22, “Radianceout” is the radiance at the output image 26, “Areain” is the area of the input source 22 and “Areaout” is the area of the output image 26. The quantities Ωin and Ωout are, respectively, the projected solid angles subtended by the input source and output image light cones. In such a lossless system, it can be shown thatRadiancein=Radianceout  [Equation 3]and(Areain)(Ωin)=(Areaout)(Ωout)  [Equation 4]If the index of refraction of the optical transmission medium is different at the input source and output image positions, the equality in Equation 4 is modified to become(nin2)(Areain)(Ωin)=(nout2)(Areaout)(Ωout),  [Equation 5]where nin is the index of refraction at the input position and nout is the index of refraction at the output position. The quantity (n2)(Area)(Ω) is variously called the “etendue” or “optical extent” or “throughput” of the optical system. In a conventional lossless optical system, the quantity (n2)(Area)(Ω) is conserved.
In U.S. Pat. No. 6,144,536, herein incorporated by reference, Zimmerman et al demonstrated that for the special case of a source that has a reflecting emitting surface, an optical system can be designed that recycles a portion of the light emitted by the source back to the source and transmits the remainder of the light to an output position. Under certain conditions utilizing such light recycling, the effective luminance of the source as well as the output luminance of the optical system can be higher than the intrinsic luminance of the source in the absence of recycling, a result that is not predicted by the standard etendue equations. In U.S. Pat. No. 6,144,536, the term “luminance” is used for brightness. As previously stated, the term “luminance” is only useful for visible optical wavelengths between 400 and 700 nanometers. Therefore U.S. Pat. No. 6,144,536 is operative only in that spectral region.
An example of a light source with a reflecting emitting surface is a conventional fluorescent lamp. A cross-section of a conventional fluorescent lamp 30 is shown in FIG. 2A. The fluorescent lamp 30 has a glass envelope 32 enclosing a hollow interior 36. The hollow interior 36 is filled with a gas that can emit ultraviolet light when a high voltage is applied. The ultraviolet light excites a phosphor coating 34 on the inside surface of the glass envelope, causing the phosphor to emit visible light through the phosphor coating 34. The phosphor coating 34 is a partially reflecting surface in addition to being a light emitter. Therefore, it is possible to design an optical system that recycles a portion of the visible light generated by the phosphor coating 34 back to the coating 34 and thereby cause an increase in the effective brightness of the conventional fluorescent lamp.
The disclosures on light recycling in U.S. Pat. No. 6,144,536 relate to linear light sources that have long emitting apertures with aperture areas greater than 100 square millimeters (mm2). Such configurations, which typically use fluorescent lamps as light sources, are not suitable for many applications such as illumination systems for large projection displays. At the application date for U.S. Pat. No. 6,144,536, a typical LED had an output of only 1 lumen per square millimeter (mm2) and a light reflectivity of less than 20%. To make an illumination system that produces 1000 lumens output for a projection display would require at least 1000 LEDs having a total LED surface area of 1000 mm2. If 1000 low-reflectivity, low-output LEDs are placed on the inside surface of a light-recycling envelope that has a 10 mm2 output aperture and that has a total inside area of 1010 mm2 (including the area of the output aperture), the overall output efficiency will be less than 2%. Less than 20 lumens from the original 1000 lumens will exit the light-recycling envelope. Such an illumination system is not very practical.
Recently, highly reflective green, blue and ultraviolet LEDs and diode lasers based on gallium nitride (GaN), indium gallium nitride (InGaN) and aluminum gallium nitride (AlGaN) semiconductor materials have been developed. Some of these LED devices have high light output, high radiance and have a light-reflecting surface that can reflect at least 50% of the light incident upon the device. The reflective surface of the LED can be a specular reflector or a diffuse reflector. Typically, the reflective surface of the LED is a specular reflector. Radiance outputs close to 7000 watts per square meter per steradian and total outputs of approximately 0.18 watt from a single packaged device are possible. Light outputs per unit area can exceed 0.045 watt/mm2. As such, several new applications relating to illumination systems have become possible. Advantages such as spectral purity, reduced heat, and fast switching speed all provide motivation to use LEDs and semiconductor lasers to replace fluorescent, incandescent and arc lamp sources.
FIG. 2B illustrates a cross-sectional view of a recently developed type of LED 40 that has an emitting layer 46 located below both a transparent top electrode 43 and a second transparent layer 44. Emitting layer 46 emits light rays 45 when an electric current is passed through the device 40. Below the emitting layer 46 is a reflecting layer 47 that also serves as a portion of the bottom electrode. Electrical contacts 41 and 42 provide a pathway for electrical current to flow through the device 40. It is a recent new concept to have both electrical contacts 41 and 42 on the backside of the LED opposite the emitting surface. Typical prior LED designs placed one electrode on top of the device, which interfered with the light output from the top surface and resulted in devices with low reflectivity. The reflecting layer 47 allows the LED to be both a light emitter and a light reflector. Lumileds Lighting LLC, for example, produces highly reflective green, blue and ultraviolet LED devices of this type. It is expected that highly reflective red and infrared LEDs with high outputs and high radiance will also eventually be developed. However, even the new green, blue and ultraviolet gallium nitride, indium gallium nitride and aluminum gallium nitride LEDs do not have sufficient radiance for many applications.
LEDs, including inorganic light-emitting diodes and organic light-emitting diodes, emit incoherent light. On the other hand, semiconductor laser light sources, such as edge-emitting laser diodes and vertical cavity surface emitting lasers, generally emit coherent light. Coherent semiconductor laser light sources typically have higher brightness than incoherent light sources, but semiconductor laser light sources are not suitable for many applications such as displays due to the formation of undesirable speckle light patterns that result from the coherent nature of the light.
It would be highly desirable to develop incoherent illumination systems based on LEDs that utilize light recycling to increase the illumination system output radiance. Possible applications include projection displays, flat-panel displays, avionics displays, automotive lighting, residential lighting, commercial lighting and industrial lighting.